The invention relates to a photomultiplier tube comprising a photocathode, focusing electrodes and a focusing multiplier structure having a large input surface relative to the photocathode and comprising at least one input dynode.
The invention can be used in the general technical field of photomultiplier tubes.
The photomultiplier tube as described in the opening paragraph corresponds to a classical type of tubes which are termed "linear focused", as described in, for example, U.S. Pat. No. 3,009,764. The focusing multiplier structure, also termed Rajchmanstructure, comprises, besides the input dynode, a plurality of dynodes which are formed and arranged in such a manner that a progressive focusing of the electron paths along the multiplier structure in ensured. The focusing reduces the difference in transit times between the stages and enables the structure and, hence, the tube to have high operating rates. Besides, the fact that the focused linear tubes have a satisfactory response-linearity as a function of the incident flux can be mainly attributed to the focusing at the various stages.
However, by means of the electron-optical system at the input of the photomultiplier tubes according to the state of the art a collection efficiency of the photoelectrons emitted by the photocathode is obtained which is not constant, because the input dynode of the focusing multiplier structure, i.e., in this case the first dynode of the tube, has a smaller surface than the photocathode and is spaced too far from said photocathode. This results in a certain degree of collection inhomogeneity, so that the electrons issuing from the periphery of the photocathode are not all captured by the input dynode.